Low coherence interferometer apparatus

ABSTRACT

Low coherence interferometer apparatus for investigation of a sample ( 15 ), in particular for multi-dimensional imaging, having an interferometer configuration ( 2 ) comprising a low coherence light source, a probe head ( 9 ) having a light exit opening ( 16 ) for irradiating light into the sample, an optical coupler, a reference reflector and a detector ( 13 ). The optical paths between the elements of the interferometer configuration ( 2 ) form interferometer arms. The optical coupler and the reference arm of the interferometer configuration ( 2 ) are integrated into a common optical chip ( 28 ). In addition to the reference reflector ( 11 ), the reference arm comprises a deflection reflector ( 33 ) formed on an end surface ( 35 ) of the optical chip ( 28 ) in such a manner that the reference light is cross-coupled between a first light guide ( 10   a ) forming a first portion of the reference arm ( 10 ) and a second light guide ( 10   b ) forming a second portion of the reference arm ( 10 ).

This application is a continuation of Ser. No. 08/875,351 filed Jul. 28,1997 now U.S. Pat. No. 6,144,449, which is a 371 of PCT/DE97/00167 filedJan. 23, 1997.

The invention concerns a low coherence interferometer apparatus for theinvestigation of a sample, in particular for multi-dimensional imagingin medical applications.

Low coherence interferometer methods are used for a variety ofapplications. They are normally referred td in the art as LCI (LowCoherence Interferometry) methods or as OCDR (Optical Coherence DomainReflectometry). The abbreviation LCI is used below for reasons ofsimplicity.

LCI methods are utilized or are at least discussed for a variety ofapplications. For example, reference can be made to the followingcitations:

1) Danielson et al: “Guide-wave Reflectometry with MicrometerResolution”, Applied Optics, 26 (1987), 2836-2842.

2) Schmitt et al: “Measurement of Optical Properties of BiologicalTissues by Low-Coherence Reflectometry”, Applied Optics, 32 (1993),6032-6042

3) WO 95/30368

4) DE 2528209 A

5) DE 3201801 A1.

6) WO 92/19930

7) DE 4204521C1

8) U.S. Pat. No. 5,073,024

All LCI methods have the common property that light from a low coherence(wide spectral band width emitting) light source is split into twopartial beams—a measuring light beam and a reference light beam. The twobeams are joined upstream of a detector to produce an interferencesignal containing the desired information. A principal component in thelow coherence interferometry apparatus (designated below as “LCIapparatus”) is an interferometer configuration comprising, in additionto the low coherence light source, an optical coupler, a referencereflector, a probe head having a light exit opening for irradiatinglight into the sample, and the detector.

The optical paths between these interferometer elements form so-calledinterferometer arms. Light from the light source passes through thelight source arm and is incident on the optical coupler where it issplit. One part, constituting measuring light, passes through a samplearm and the probe head and is irradiated into the sample. The secondpart of the light, constituting reference light, passes through areflector arm and is incident on the reference reflector. Both fractionsof the light are reflected (the measuring light in the sample and thereference light at the reference reflector) and are guided back to theoptical coupler along the same optical path (sample arm and referencearm respectively) where they are joined together and introduced throughthe detector arm to the detector. The light-sensitive surface of thedetector can measure an interference signal caused by the interferencebetween the two parts of the beam.

In order for an interference to occur, the optical path length in thereference arm (between the optical coupler and the reference reflector)differs by at most the coherence length of the light source from theoptical path length of the measuring light between the optical couplerand the point of reflection in the sample. An interference signal ismeasured only if this condition is fulfilled. This fact is utilized tolimit the investigation to one particular measuring depth, designatedbelow as the LCI measuring depth, through appropriate adjustment of thelength relationships between the reference arm and the sample arm.

This fundamental principle of the LCI measuring technique is used toallow various applications through variation of certain measurementdetails and through analysis of the interference signal.

For example, reference 1) concerns the investigation of the structure ofoptical fibers, in particular for localizing optical defects. References2) and 3) concern various aspects of investigations in biological tissue(in particular skin tissue). These authors are only concerned withobtaining information in dependence on the LCI measuring depth definedby the interference criterion. These publications therefore perform apure depth scan (also termed “longitudinal scan”), i.e. the length ofthe reference arm is varied to adjust the LCI measuring depth.

In contrast thereto, references 4) through 6) describe methods andapparatuses with which an additional lateral scan is carried out inorder to obtain in various ways a picture of the distribution of theinformation of interest in the lateral direction (parallel to thesurface of the sample). These methods therefore pertain tomulti-dimensional imaging. In addition to a depth scan, a scan in atleast one transverse direction (“lateral scan”) is carried out. Theinvention is particularly concerned with methods and apparatuses formulti-dimensional imaging using the LCI principle (Optical CoherenceTomography (OTC)).

References 4) through 6) relate to OTC methods. Reference 4) pertains toa surface scan of a manufactured product and reference 5) concernsinvestigation of the eye, in particular the retina. Reference 6)provides particularly detailed discussion of multi-dimensional imagingof a sample using the LCI technique, in particular—as in the presentinvention—for applications involving the investigation of biologicalsamples. The most important practical example discussed in reference 6)is investigation of the eye (as in reference 5)). The present inventionprimarily concerns investigations of samples having very finelydistributed structures, in particular human skin.

The OTC method has particular advantages over other imaging methods (forexample, ultrasound imaging, X-ray CT and lateral scanning confocalmicroscopes), since it does not utilize ionizing radiation and istherefore not damaging and since it facilitates high image resolution.It is particularly well suited for the investigation of relatively finestructures near to the surface. For the case of skin, the current stateof development permits a maximum LCI measuring depth of approximately1.5 mm. A spatial resolution better than 10 μm is possible in both theaxial and lateral directions.

The methods known in the art through references 1) through 6) aredifficult to perform and require a large amount of space in the range ofthe probe head. For these reasons, LCI apparatus have been proposed withwhich portions of the interferometer configuration (at least the opticalcoupler and the reference arm) are both integrated into an optical chip(references 7) and 8).

An optical chip is an optical element made from a transparent material(normally glass) having integrated light guides. The light guides aremade from a material having an index of refraction which is greater thanthat of the remaining optical chip. As is the case with optical fibers,the optical waveguide properties of the light guides integrated into theoptical chip result from total internal reflection at the refractiveindex interface. Optical chips (also designated as integrated opticalcomponents) are primarily used for optical data transfer in opticalfiber communication systems. Further details are given in comprehensivereferences such as “Optical Waveguides Advance to Meet FiberopticDemands”, by E. D. Jungbluth, Laser Focus World, April 1994, 99-104 orthe book “Integrated Optics”, Proceedings of the Third EuropeanConference, ECIO'85, H.-P. Noltes, R. Ulrich (Editors); Springer Verlag1985 in which an article by P. O. Andersson et. al., “Fiber OpticMach-Zehnder Interferometer Based on Lithium Niobate Components” ispublished on pages 26 through 28.

The measuring arm of an interferometer configuration always includes apart which is outside of the chip, namely the optical path between thelight exit opening of the chip and the point of reflection in thesample. For this reason, the reference arm, which is completelyintegrated in the chip, is substantially longer than that portion of themeasuring arm travelling through the chip. Since both arms depart fromthe same optical coupler, the reference arm cannot travel in a straightline through the optical chip. At least one beam deflection is necessaryin order to accommodate the additional length by means of a zigzag ormeandering travel of the reference arm in the optical chip.

Reference 7) describes two fundamental possibilities for effecting sucha deflection. Firstly, a meandering curved optical waveguide can be usedfor the reference arm. Secondly, in addition to the reference reflectordisposed at the end of the reference arm, at least one further reflector(“deflection reflector”) can be provided to reflect the reference lightfrom a first light guide into a second light guide. The light guidesthereby constitute a first and a second partial path of the referencearm. The deflection reflector is conventionally formed by a slit in thewaveguide produced by reactive ion etching. Since this manufacturingstep is quite difficult, publication 7) considers the use of adeflection reflector to be disadvantageous and prefers circular-shapeddeflection without reflectors.

In view of the above, the present invention is characterized in that thedeflection reflector is provided on an end surface of the optical chipin such a manner that the reference light is cross-coupled between thetwo light guides which constitute portions of the reference arm.

The invention facilitates an extremely compact and simple assembly of aplurality of closely spaced interferometer configurations. In thisfashion, multi-channel investigation of the, sample is possible with themeasuring light being irradiated into the sample at a plurality ofclosely spaced entrance locations of the interface. In consequencethereof, the integration time necessary for imaging with a particulardesired resolution is reduced or an improved optical spatial (transverseand/or longitudinal) resolution is achieved for a given integrationtime.

It is easy to manufacture a reflector on the end surface of an opticalchip. In principle, an arbitrary material having an index of refractiondiffering from that of the light guide can be applied onto the interfaceto cause reflection via a discontinuous change in the index ofrefraction. Metallic mirroring is however preferred for intensityreasons.

In order for the interferometer configuration to function, the lightmust cross-couple between the two light guides. That is to say, thelight from one of the light guides which is incident on the reflectingsurface should enter nearly completely (at least approximately 90%) intothe other light guide and not be reflected back into the same lightguide. Within the framework of the invention, several configurations areproposed in order to guarantee this cross-coupling.

For example, the cross-coupling can be achieved if both light guides areincident on the deflection reflector at an (identical) acute angle. Aminimum angle (e.g. approximately 5°) relative to the normal to thesurface is necessary in order to prevent disturbing reflection ofportions of the light back into the same waveguide. In order to minimizeintensity losses, it is necessary that in such an arrangement thedeflection reflector is localized with very high precision at thecrossing point of the two light guides. Procedures for accomplishingthis will be described further below.

In accordance with a second configuration, complete cross-coupling ofthe reference beam from the first light guide into the second lightguide is effected in that both waveguides travel parallel to another ina light coupling arrangement along a coupling length L immediatelybefore the deflection reflector. The coupling length L is selected toeffect cross-coupling. Towards this end, a phenomenon is used which isknown in the art of light couplers and described in the publication

9) K. J. Ebeling: “Integrierte Optoelektronik”; Springer Verlag 1992,second edition, pages 146-151

Two light guides are in a “light coupling arrangement”if they travelparallel to one another at a separation sufficiently small to effectcoupling via mutual penetration of evanescent waves. A completecross-coupling or switching over of the incident signal opticalintensity is observed at a particular coupling length L_(c) (“crossingcondition”). At other lengths, varying portions of the light remainwithin the same light guide (“noncrossing condition”). This is describedin more detail below.

In general, the invention facilitates relatively simple production ofcompact interferometer configurations in optical chips. The technicalproblems associated with arrangement of the deflection reflector on anend surface of the optical chip can be solved in a manner allowingeconomical manufacture.

The separation between the probe head and the sample interface is variedfor depth scanning. Towards this end, it is advantageous to focus thelight penetrating into the sample with the assistance of an opticalsystem disposed between the probe head and the sample interface in sucha manner that the separation between the focal point inside the sampleand the interface (designated below as “focus depth”) coincides with theLCI measuring depth.

One thereby has the problem that the focus depth and the LCI measuringdepth change differently when the separation between the probe head andthe interface is changed. If the sample has an index of refraction N andthe probe head is displaced by an amount z towards the sample, the LCImeasuring depth is displaced to by a somewhat lesser amount, namely byz_(i)″=z/N. In contrast thereto, the same displacement of the probe headby an amount z causes an increase in the focus depth by approximatelythe factor N: z_(f)′=z*N due to refraction, at the interface. Despitethis effect, a second principal feature of the invention, which ispreferentially utilized in combination with the first principal featurebut also has independent significance, provides for focus correctionmeans to achieve agreement between the focus depth and the LCI measuringdepth within the entire longitudinal scan range. These focus correctionmeans guarantee that the focus depth changes equally along with changesin the LCI measuring depth. In this manner particularly good opticalimaging quality is achieved within the entire depth scan range.

The invention is more closely described below with reference to theembodiments schematically represented in the figures.

FIG. 1 shows a schematic representation of an LCI reflectometeraccording to prior art,

FIG. 2 shows a cut through the scan module of an LCI reflectometer inaccordance with the invention,

FIG. 3a shows the waveguide pattern of a first embodiment of the opticalchip,

FIG. 3b shows an enlarged view of a portion of FIG. 3a,

FIG. 4 shows the waveguide pattern of a second embodiment of the opticalchip,

FIG. 5a shows the waveguide pattern of a third embodiment of the opticalchip,

FIG. 5b shows an enlarged section of FIG. 5a,

FIG. 6 shows the waveguide pattern of a fourth embodiment of the opticalchip,

FIG. 7 shows a first embodiment of an optical system having focuscorrection means in accordance with the invention,

FIG. 8 shows a second embodiment of an optical system having a focuscorrection means in accordance with the invention,

FIG. 9 shows an embodiment similar, to FIG. 6 however with enlargedoptical aperture,

FIG. 10 shows a section of an embodiment alternative to FIG. 9,

FIG. 11 shows an additional embodiment based on FIG. 8.

The LCI reflectometer 1 in accordance with prior art shown in FIG. 1consists essentially of an interferometer configuration 2 and anelectronic measuring and analysis unit 3. The interferometerconfiguration 2 comprises four interferometer arms: a light source arm 5between a light source 6 and an optical coupler 7, a sample arm 8between the optical coupler 7 and a probe head 9, a reference arm 10between the optical coupler 7 and a reference reflector 11, and adetector arm 12 between the optical coupler 7 land the detector 13.

As mentioned, in order for an interference to occur, the optical pathlength of the measuring light between the optical coupler 7 and areflection point R located within the sample 15 at an LCI measuringdepth t below the interface 17 must correspond to the optical pathlength of the reference light between the optical coupler 7 and thereflecting surface of the reference reflector 11 (to within thecoherence length of the light). If this interference condition isfulfilled, the detector measures an interference signal containinginformation about the sample at the measuring depth t. Phase modulationis often used in order to separate the interference signal fromdisturbing effects. For example, a light guide wound about a PZT (Piezotransducer) 22 can be provided in the sample arm for modulating theoptical path length with a modulation frequency. Analysis of themeasured signal of the detector 6 is done in a frequency selectivemanner using this modulation frequency.

In order to facilitate a depth scan in the sample 15, it is necessaryfor either the length of the reference arm 10 between the opticalcoupler 7 and the reference reflector 11 or the separation d between thelight exit opening 16 of the probe head 9 and the interface (surface) 17of the sample 15 to be variable. The first possibility (which has beenused most often up to this point in time) is indicated in FIG. 1 with adashed double arrow 18. In accordance with the invention, the separationd between the probe head 9 and the sample surface 17 is varied.Positioning means 19 serve for adjustment of the separation d forcontrolled and defined adjustment of the probe head 9 position relativeto the sample 15. The unit comprising the probe head 9 and thepositioning means 19 forms a scan module 20.

The light source 6, the detector 13, the PZT 22 and the positioningmeans 19 of the scan module 20 are connected to the electronic measuringand analysis unit 3. This unit contains conventional means for powersupply to the light source 6, for processing the signal of detector 13and for driving the positioning device 19.

Further description of the LCI method is given in the above mentionedreferences including description of extraction of various informationconcerning the sample from the interference signal. In the presentinvention conventional methods are used for processing and analysis ofthe measured signal. Reference therefor is made to citations 1) through8).

FIG. 2 shows a scan module 20 for effecting the first principal aspectof the invention. A probe head 9 is disposed in the scan module 20 insuch a manner that its location relative to the sample 15 is adjustablein both the longitudinal (perpendicular to interface 17, arrow 24) aswell as the lateral (parallel to the interface 17, arrow 25) directions.The positioning means 19,27 necessary therefor are schematicallyindicated in the figure. These can e.g. comprise a conventionalelectromagnetic drive. An image slice through the sample along thelateral scan line can be realized using a one dimensional lateral scan.In the event that the positioning means 27 are adapted for twodimensional motion parallel to the interface 17, information can beobtained concerning that partial volume of the sample located beneaththe scanned surface.

An optical chip 28 having waveguides 29 and constituting the main partof an interferometer configuration 2 is disposed within the probe head.Also belonging to this interferometer configuration are the light source6 and detectors 13 which are disposed in such a manner that the lightcan be irradiated into and out of the waveguide paths 29 and such thatthe exiting light can be detected. Instead of the direct inward andoutward irradiation shown here, transport from the respective lightsource and to the respective detector can often advantageously beeffected via optical fibers (“fiber coupling”).

Since the scanning should be as rapid as possible, the acceleration ofvarious masses associated therewith can lead to disturbing vibrations.In order to avoid this, it can be advantageous to provide for acounterweight (not shown) within the scan module 20 whose masscorresponds to the moving mass of the optical chip 28 and which is movedin opposition to the optical chip.

FIG. 3a shows the waveguide pattern of an optical chip 28 having themain portions of the interferometer configuration, in particular theoptical coupler 7 and the reference arm 10, both integrated into thechip. The part of the interferometer configuration integrated in thechip 28 is designated “interferometer module”. In the preferredembodiment shown, a plurality of parallel interferometer modules I₁ . .. I_(n) are provided for in the same chip for multi-channelinvestigation of the sample. The measuring light enters at a pluralityof entrance locations E₁ . . . E_(n) and is reflected at a plurality ofreflection locations R₁ . . . R_(n) (FIG. 2). Each interferometer moduleI has a light input location 30 at which the light from a light source(not shown)—advantageously transported via an optical fiber—is coupledin. An output location 32 is disposed at the same front surface 31 ofthe optical chip 28 for coupling-out the light transported in thedetector arm 12 to a detector (not shown).

The reference arm 10 must be longer than that part of the sample arm 8integrated in the optical chip between the optical coupler 7 and thelight exit opening 16. In addition to the reference reflector 11disposed at the end of the reference arm 10, a deflection reflector 33is therefore provided in the reference arm to deflect the referencelight in the optical chip 28 in the opposite direction. In theembodiment shown in FIG. 3a, both reflectors 11,33 are mirrored endsurfaces 34 on polished front surfaces 31,35 of the chip 28.

FIG. 3b shows an enlarged section in which, in this embodiment, thecross-coupling between the light guides 10 a and 10 b is effected byhaving same be incident on the deflection reflector 33 at an acuteangle. The angle α with respect to the normal to the surface N is thesame for both waveguides and should assume a value of at least 5° toguarantee sufficient complete cross-coupling. The longitudinal positionof the deflection reflector 33 in the direction of the double arrow 36must be located precisely at the crossing point of the light guides. Theprecision required for this positioning depends on individualrequirements for the completeness of the cross-coupling and the extentto which slight losses in intensity can be tolerated. A requiredprecision of +/−10 μm can be given as a guideline.

The number of interferometer configurations I₁ . . . I_(n) and thelateral separation of the light exit openings 16 can be chosen inaccordance with the actual requirements. A particular advantage of theinvention is that a very close separation is possible between light exitopenings 16 and consequently between reflection points R₁ . . . R_(n) inthe sample 15. Within the framework of LCI imaging, it is therebypossible to study a given investigation surface (FOV, “Field of View”)with little or even no lateral mechanical motion. For example, a FOV of1×1 mm² with 16 interferometer modules leads to a separation betweenlight exit openings 16 of 62.5 μm at the front surface 25 of the opticalchip 28 facing the sample. In consequence thereof, an optical resolutioncorresponding to this separation can be achieved without mechanicalmotion. Since conventional optical fibers have diameters ofapproximately 250 μm , the separation between the input locations 30 andthe output locations 32 on the irradiation input front surface 31 of theoptical chip must be larger so that a compression of the waveguide pathswithin the optical chip 28 is necessary. Equal lengths of the mutuallycorresponding waveguide paths in the interferometer modules I₁ . . .I_(n) can be easily effected through appropriate bending of the paths.

Even higher resolution can be achieved by laterally displacing theoptical chip (as shown in FIG. 2). It is thereby also very advantageousif only a small lateral displacement (in the example given approximately60 μm is required. This can be accomplished with a high degree ofreliability and speed using special piezo elements (“stacked piezoelements”).

In general, the invention thereby facilitates a very dense configurationof the exit openings 16 of a plurality of interferometer configurationsin one probe head 9. Applications of interest have opening separationsof preferentially less than 0.5 mm.

As has already been stated, when producing an optical chip in accordancewith FIGS. 3a and 3 b, the reference reflector 33 must be preciselypositioned at the crossing point of the light guides 10 a and 10 b. Thisrequires an extremely precise cutting to length of the front surfaces 31and 35 of the optical chip 28.

FIG. 4 shows one possibility for allowing this cutting. The optical chip28 includes two auxiliary interferometer modules H₁, H₂ in addition tothe interferometer modules I₁ . . . I_(n) (configured in the same manneras in FIG. 3). These are used for extension in the following manner.

In order to fulfill the interference condition in the auxiliaryinterferometers, the lengths of their sample arms 38 must correspond tothe lengths of their reference arms 40. Due to the curved path, thereference arm 40 has a somewhat larger length than the sample arm 38. Anelement having a discontinuous change in refractive index is located atits end to form a (weak) reflector 41 at a position which exactlycorresponds to the desired nominal position of the front surface 35. Aninterference signal is thereby produced by even a very small increase inthe refractive index. When the front surface 35 is then ground downduring the course of manufacture, an interference signal peak resultswhen the length of the sample arm 38 coincides with the length of thereference signal 40. In this manner the nominal position of the frontsurface 35 can be precisely maintained. The second auxiliaryinterferometer H₂ facilitates a precise correction of the angularposition of the front surface 35.

FIG. 5a shows a section of the waveguide pattern of an alternativeembodiment of an optical chip 28, wherein only one single interferometerI1 is shown in this case. However, as in FIG. 3a, additionalinterferometer modules are preferentially integrated within the sameoptical chip 28.

In this embodiment, a sufficient cross-coupling between the light guides10 a and 10 b is achieved by—as can be more clearly seen in FIG.5b—having them travel parallel to another along a coupling length L in alight coupling device. The separation is sufficiently close (typicallycirca 15 μm) that mutual coupling occurs via evanescent waves. Thecoupling length L is selected to effect cross-coupling. As described inthe above mentioned reference 9), complete cross-coupling is achieved atparticular coupling lengths L_(c). At intermediate lengths, varyingfractions of light remain in the same light guide. The following formulahas been given for the coupling lengths L_(c) (in lower portion of page149):$L_{C} = {\frac{1}{Kappa}\left( {{2\quad {Ny}} - 1} \right)\quad {\pi/2}}$

with Kappa the coupling factor, and Ny an intger. The shortest lengthfor complete cross-coupling is $L_{C} = \frac{\pi}{2{Kappa}}$

The configuration in accordance with the invention is different fromthat of the reference in that the deflection reflector which is orientedperpendicular to the two light guides reflects the light back into them.Complete cross-coupling requires the coupling length L along which thelight guides travel parallel to another to be one half of Lc: L=Lc/2.

In practice, the coupling length L is empirically adjusted to maximizecross-coupling. This geometry has the substantial advantage that thedegree of cross-coupling changes relatively little in dependence on theposition of the reverse reflector so that the degree of precisionrequired for positioning the reverse reflector is rather low.

An additional distinguishing feature of the embodiment shown in FIG. 5ais the use of a chirped grating 42 as the reference reflector 11. Achirped grating is a grating structure having a continuously increasingor decreasing grating constant. In this manner, light of shorterwavelength is reflected at the sections having smaller grating constantsand longer wavelength light is reflected from sections having largergrating constants. Therefore, reflection from a chirped grating leads todiffering fractions of light (necessarily polychromatic due to the lowcoherence) being reflected at differing locations of the grating. In theembodiment shown, e.g. the short wavelength portion of the light isreflected first at grid 42 (the grating constant increases in thedirection of the incident light).

One can therefore take advantage of the properties of a chirped gratingto compensate for optical dispersion. Optical dispersion is a problem ina high resolution LCI configuration, since the measuring light and thereference light are in part transported through differing media havingdiffering dependencies for the light velocity and therefore for theindex of refraction on the wavelength (dispersion). Therefore, theoptical paths (product between the geometric path and the index ofrefraction) have differing lengths for differing wavelengths. This leadsto a smearing-out of the interference signal and thereby to worseresolution in the longitudinal direction. Appropriate selection of thechirp ramp of the grating 42 of a chip grating can, to a goodapproximation, cause that all light portions travel the same opticalpath length as the measuring light independent of their wavelength,thereby to optimize the resolution.

Whether or not a chirped grating is used, it can be advantageous todispose the reference reflector (e.g. in the form of ion etchedstructure) at another location on the optical chip rather than at theend surface of the optical chip (as in FIG. 3a).

The optical chip shown in FIG. 6 contains a modified interferometerconfiguration having an additional compensating optical path 45 forsuppressing relatively large DC voltage portions of the detector signal.The compensating optical path 45 leads from an additional opticalcoupler 46 via a reflector 48 to an additional output location 49 on thefront surface 31. In the embodiment shown, an additional input location44 is also provided for input of light from another light source havinge.g. a differing wavelength.

The light exiting at output locations 49 and 32 is detected by twodetectors (not shown) connected to a compensation-circuit. Thisconventional compensation technique is described e.g. in WO 92/19930.FIG. 6 shows the manner in which one can, in accordance with theinvention, integrate the compensating optical path into the same opticalchip in which the reference optical path travels.

FIGS. 7 through 9 show differing embodiments of focussing opticalsystems suitable for LCI reflectometers in which the depth scanning iseffected through variation of the separation d between the probe head 9and the interface 17. Focus correction means 54 are provided in eachcase to guarantee that the focus depth in the sample 15 always changesequally with changes in the LCI measuring depth t. In this manner, asharp focus is achieved at each point of the depth scan.

FIG. 7 shows a probe head 9 preferentially comprising an optical chip 28in accordance with one of those in FIGS. 2 through 6. An optical system55 comprising a plurality of lenses 50 through 53 causes each light exitpoint 16 of the probe head 9 to be imaged in a focal plane 56. In orderto guarantee optimal optical resolution, the reflection point R to whichthe LCI scan is adjusted should lie in the focal plane 56 of the opticalsystem 55 for the entire scan range. This is not the case in the absenceof additional focus correction means, since (for an index of refractionN>1, which is always the case in practice) a displacement of the probehead 9 by an amount z leads to a different displacement of the focalplane 56 and the reflection point R (in other words, to differentchanges in the focus depth and the LCI measuring depth). The LCImeasuring depth changes less than z and the focus depth by more than z.

In the embodiment shown in FIG. 7, the focus correction is effected viaa fluid-filled, transparent bubble 57 disposed in the optical path ofthe measuring light between the probe head 9 and the sample 15 in such amanner that its thickness decreases when the probe head is moved towardsthe sample. The fluid in the bubble 57 should thereby have an index ofrefraction as similar as possible to the index of refraction of thesample 15. Movement of the probe head 9 by an amount z causes thethickness D of the bubble 57 to decrease by the same amount to displaceboth the focal plane 56 as well as the LCI reflection point R in thesample towards the right.

FIG. 8 shows a particularly preferred embodiment with which the focuscorrection is achieved using purely optical means. Here, the opticalfocus correction system 60 comprises two lenses 61, 62 whose separationis as large as the sum of their focal lengths f1 and f2. A configurationof this kind is used in Keppler telescopes and is designated as aKeppler system. The lenses 61 and 62 can be simple lenses or multi-lenssystems.

The focus correction results when the focal lengths f₁ and f₂ of lenses61 and 62 are chosen in consideration of the index of refraction N inthe sample according to the formula f₂/f₁=1/N. This geometric imagingcondition leads to a correction in the depth direction by a factor 1/N².In this manner, a displacement of the probe head 9 by z no longer leadsto an extension in the sample by a factor N, rather to shortening by thefactor 1/N²*N=1/N. The focus depth therefore changes equally with theLCI measuring depth which is likewise shortened by 1/N.

In an embodiment of this kind an additional improvement in focussing ispossible if one of the lenses in the Keppler system is adjustable by asmall amount (for example up to 20 μm) in the longitudinal direction(double arrow 67) with the assistance of a positioning drive 65 (e.g. apiezo drive). When focussing, it is thereby possible to take intoconsideration the fact that the index of refraction changes in differinglayers of skin so that the average index of refraction is not constantfor changes in the LCI measuring depth t. Adjustment of a lens effectsappropriate compensation in each case.

In FIGS. 7 and 8, there is only one optical system 55 or 69 for allinterferometer modules so that all light exit openings of the probe head9 are imaged in the plane 56. Only one optical path is shown for reasonsof clarity.

In order to compensate for correction errors in the relatively largelenses of optical systems 55 and 60, individual small lenses 63 (dashedrepresentation in FIG. 8) can be advantageously disposed in front ofeach light exit opening 16 in such a manner as to compensate for imagingdistortions of the large lenses.

In the embodiment of FIG. 8, the relationship between the focal lengthsof the two lenses 61 and 62 determines the optics and thereby themagnification. Therefore the optical aperture on the sample side cannotbe easily enlarged for a given relatively small aperture at the inputside of the optical system. A large optical aperture on the sample sidecan however be desirable, in particular for improving the opticalresolution of the depth scan.

In order to accomplish this, an additional optical imaging system 70shown in FIG. 9 can be advantageously provided upstream from the opticalsystem 60 in accordance with FIG. 8, this additional optical systembeing movable together with the probe head 9. This system produces ascaled down image of the light exit opening 16 and of the probe head 9in the object plane of the Keppler system (i.e. in the input side focalplane of lens 61). In this manner, the aperture angle α is increased atthe input side of the focus correction system 60 and thereby theaperture angle β on its sample side.

The same effect of the additional optical imaging system 70 can, asshown in FIG. 10, be achieved using a concave lens 74 rather than aconvex lens 72. In this case, a scaled down virtual image is produced ina plate 75 at that side of the concave lens facing the probe headinstead of the real image in the plane 71 shown in FIG. 9. The purposeof lenses 72, 74 can also be achieved by micro-lenses which (such aslenses 63 in FIG. 8) are each associated with one interferometerconfiguration.

The optical systems 55 and 60 are disposed in a stationary manner in aparticular position at the interface 17 of sample 15. Only the probehead 9 must be moved for scanning. In accordance with the invention, thefocussing optical system should generally be substantially stationary toeffect as small an amount of moving mass as possible. This does not ofcourse preclude a micro-positioning of lenses as e.g. effected bypositioning drive 65 in FIG. 8. Such a positioning (of up to at most0.02 mm) can be rapidly and precisely effected using e.g. piezoelectricelements.

Within this context, especially the actual focus correction systemshould be “stationary”, i.e. the Keppler system 60 for the case of FIGS.8 through 10. Movement of the optional upstream correction lenses 63,72, 74 together with the probe head 9 is much easier to realize, sincethese lenses are rather small and of low mass. The mass fraction of theoptical elements of the entire imaging system which move together withthe probe head 9 should however be as small as possible (less than 20%,preferentially less than 5%).

The embodiments in accordance with FIGS. 8 through 10 facilitate aparticularly simple realization of the lateral scanning needed for anOCT system. FIG. 11 shows that a pivoting mirror 69 can be disposedbetween the lenses 61 and 62 in such a manner that the required lateraldisplacement of the image can be effected with a very small pivot motionto optically scan.

What is claimed is:
 1. An optical coherence tomography apparatus formulti-dimensional imaging of a sample comprising: a plurality ofparallel disposed interferometer modules for multi-channel investigationof the sample, each interferometer module comprising a low coherencelight source; a light exit opening for irradiating light into thesample; a detector; an optical coupler for joining a measuring light anda reference light in such a manner that both are incident on thedetector at the same location to produce an interference signal suchthat light reflected at an LCI measuring depth beneath the interface ofthe sample is selectively detected; a reference reflector; a lightsource arm providing an optical path between the light source and theoptical coupler and for irradiating measuring light into the sample; asample arm providing an optical path between the optical coupler and thesample and for guiding reflected light from a reflection point insidethe sample to the detector via the optical coupler; a detector armbetween the optical coupler and the detector and for guiding reflectedand reference light to the detector; and, a reference arm providing anoptical path between the optical coupler and the reference reflector andfor guiding reference light therebetween, said reference arm beingcommonly integrated with the optical coupler in an optical chip whichincludes a first light guide forming a first partial path of thereference arm and a second light guide forming a second partial path ofthe reference arm and a deflection reflector between the first partialpath and the second partial path and formed at an end surface of theoptical chip such that the reference light traveling from the opticalcoupler to the reference reflector and traveling from the referencereflector to the optical coupler is cross-coupled between the two lightguides forming the first and second partial paths when traveling fromthe optical coupler to the reference reflector and when traveling fromthe reference reflector to the optical coupler; wherein an optical pathlength traveled by the reference light inside the optical chip alongboth partial paths of the reference arm between the optical coupler andthe reference reflector is longer than the section of the sample armbetween the optical coupler and the light exit opening but the opticallight path length of the total measuring light path between the opticalcoupler and the reflection point inside the sample does not differ fromthe optical light path length of the reference arm by more than thecoherence length of the low coherence light source.
 2. Apparatusaccording to claim 1, characterized in that the deflection reflector(33) is formed by a mirrored end surface (34) of the optical chip. 3.Apparatus according to any one of claims 1 or 2, characterized in thatthe first light guide (10 a) and the second light guide (10 b) areincident on the deflection reflector (33) at an acute angle α tocross-couple the reference light.
 4. Apparatus according to any one ofclaims 1 or 2, characterized in that the first light guide (10 a) andthe second light guide (10 b) travel parallel to one another in a lightcoupling device along a coupling length L directly before the deflectionreflector (33), wherein the coupling length L is adapted to effectcross-coupling.
 5. Apparatus according to claim 1, characterized in thatthe optical chip (28) in the reference arm (10) comprises a chirpedgrating (42).
 6. Apparatus according to claim 1, characterized in thatthe optical chip comprises an interferometer module having an additionalcompensating optical path (45) a portion of the light from the lightsource (6) being branched-off by means of an additional optical coupler(46) and guided to an additional light detector for compensation ofportions of light which are incapable of interference.
 7. Apparatusaccording to claim 1, characterized in that positioning means (27) areprovided for lateral scanning to change the position at which themeasuring light is irradiated through a interface (17) into the sample(15) in at least one spatial direction parallel to the interface (17).8. Apparatus according to claim 7, characterized in that positioningmeans (19) are provided to change the separation (d) of the probe head(9) from the interface (17) for depth scanning.